chore: improve pull request template

Author: ldez

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,3 +8,12 @@ The updates happen at least automatically once a week (Sunday 11am UTC).
 No pull requests to update a linter will be accepted unless you are the author of the linter AND specific changes are required.
 
 -->
+
+<!--
+
+WARNING:
+
+Pull requests from a fork inside a GitHub organization are not allowed.
+Only pull requests from personal forks are allowed. 
+
+-->